1. Field of the Invention
This invention relates to cross-linked acid gels useful as fracture acidizing fluids and as workover and completion fluids for use in oil and gas wells in subterranean formations, to a novel encapsulated breaker for such fluids and a method of treating subterranean formations and wells using such cross-linked fluids containing said novel breaker.
The art of well stimulation commonly employs a technique called fracture-acidizing to enhance the recovery of either oil or gas from subterranean formations. Normally, fracture-acidizing involves the injection of an aqueous acid, which may or may not contain a proppant material, into a well bore at such a rate and pressure as to exceed the formation stresses, thereby causing rock fatigue and inducing new fractures in the formation. Fractures are natural or induced cracks or channels in the formation matrix. Stimulation by this technique is achieved by allowing the acid to etch the fracture face. Since the face is a heterogeneous composition, the acid reaction rates will vary on the exposed surface. After the exerted injection pressure has been relieved, fracture closure will occur but the fracture face is no longer uniform and in most cases will not perfectly realign due to the etching action of the acid. If a proppant is pumped with the acid, areas not etched by the acid will be "propped" open by the proppant material.
In each case, a more conductive channel is provided to allow the oil or gas to flow to the well bore after the injection pressure is relieved. When an aqueous acid is injected into a well bore in a fracture-acidizing application, it is often advantageous to use a viscosifying or gelling agent in the fluid. Viscous fluids possess several properties which are favorable to fracture acidizing. For example, the fluid viscosity is proportionally related to the created fracture volume and fracture width. Higher fluid viscosities, therefore, will generate larger fracture volumes and fracture widths. In addition, viscous fluids decrease the rate at which acid is exposed to the formation, allowing the acid to penetrate more deeply into the fracture before it is spent. Viscous fluids further serve as efficient proppant transporting media necessary to place a proppant into the etched fracture.
2. Prior Art
Various materials are known to act as viscosifying or gelling agents for fracture acidizing fluids. For example, guar gum, hydroxypropyl guar, hydroxethyl cellulose, carboxylmethylhydroxyethyl cellulose, xantham gum and acid stable polymers and copolymer compositions have been used.
Normally, an acid such as hydrochloric, hydrofluoric, formic, acetic and mono-, di- and tri-chloroacetic acids are used in these compositions. The aqueous acid, in a strength of from about 3% to about 28% or more by weight of the total fracture acidizing fluid, is added to the gelling agent and the composition is pumped down hole. Unfortunately, many of the gelled fracture acidizing fluids are not stable at the conditions encountered down hole, wherein temperatures of 50.degree. C. to 90.degree. C. and higher are encountered, combined with the strongly acid conditions inherent in the system. Because of the strongly acid, high temperature conditions, many of the gelled fracture acidizing fluids break down, lose their viscosity and completely release the acid before the fluid has been pumped through the formation.
Recently, cross-linked acidizing polymer gels have found increasing acceptance as viscosifying/gelling agents because of their greater ability to maintain their viscosity under down hole conditions and decrease the reaction rate of the acid with the formation.
The acid systems containing cross-linked polymers must have the characteristic, common with most acid systems gelled with natural and synthetic polymers, of eventually breaking down after most of the acid has been spent so that the fluid may be pumped out of the formation.
It is known that cross-linked gelled acid fracture acidizing fluids, wherein a polymer is cross-linked with titanium or zirconium compounds, are sensitive to fluoride, phosphate or sulphate anions. The fluoride, phosphate or sulphate anions coordinate with the titanium or zirconium cross-linking agent and renders it incapable of coordination with groups along the polymer chain, thereby affecting a reversion of the polymer to the non-cross linked state. Such reversion immediately lowers the viscosity of the acid gel system, releasing the acid and rendering the fluid capable of being pumped out of the formation and/or well and back to the surface.
U.S. Pat. No. 4,604,218 describes a cross-linked gelled aqueous acid system for use in fracture acidizing wherein a copolymer of an alkyl allyl ammonium halide and a vinyl phosphoric acid is cross-linked with a titanium or zirconium compound. Control of the viscosity of the fracture-acidizing gel is obtained by incorporating in the system a gel degrading substance, e.g., a compound containing fluoride, phosphate or sulphate anions which, as described above, coordinates with the cross-linking agent and thereby "breaks" the viscosity of the gel so that it decreases to a low viscosity to allow return of the fluid to the well bore. It is stated that the rate of fluoride release can be better controlled by encapsulating the gel degrading substance with synthetic polymers or natural gums. There is no disclosure, however, of such an encapsulated breaker or of a system which contains an encapsulated fluoride, phosphate or sulphate anion which would accomplish the breakdown of the viscosity of the cross-linked aqueous acid system over time. As described in column 7 thereof, a cross-linked system containing a non-encapsulated gel degrading substance exhibited viscosity decline at the low temperature of only 23.degree. C. over six (6) hours. Further, heat stability tests of the cross-linked system (col. 7) at 82.degree. C. were conducted in the absence of a gel degrading substance and under static conditions and cannot be considered an indication of stability of a system incorporating a breaker under actual use conditions involving considerable shear stresses on the cross-linked system. In column 8 it is stated that the fluid viscosity of the cross-linked system can be reduced over a period of time by the presence of fluoride, phosphate or sulphate groups which eventually degrade the cross-linked structure so that the gelled system returns to a lower viscosity. There is no disclosure, however, of how one skilled in the art could accomplish such delayed degradation.